Firstly, it should be noted that for clarity, the disadvantages of the prior art are presented for the case of the GSM standard. However, aspects of this disclosure apply to all types of cellular networks, such as those for example defined by the 3GPP (third generation partnership project). A person skilled in the art could thus easily implement this disclosure in a UMTS or other type of network.
The growing popularity of the GSM system throughout the world has led operators to deploy this service not just in the metropolitan regions but also increasingly in rural and more isolated or distant zones. In the latter type of region, the land infrastructure is often inadequate or poorly adapted to provide good network coverage. A satellite radio-communication system is in this case a very good means of extending the GSM network and this type of system is today widely used in many regions of the world.
However, the satellite radio resources still remain expensive, and the problem of this type of application lies in the techniques of reducing bandwidth required for the transmission of data and improving the quality via satellite radio.
Such a problem remains valid in the case where two users are located in a same geographical cell, or at least located in geographical cells that are close to one another. In such a case, it can be understood that classic GSM network transmission techniques, which in essence are centralised, consume traffic resources that are considerably higher than if they were optimised.
To make this clearer, the disadvantages of the prior art will be described below in the specific case of a GSM system implemented via a satellite connection, and in which two users located in a same cell or in two cells that are sufficiently close in the GSM network, downstream of the satellite connection, are in communication.
1.0 GSM Architecture
In relation to FIG. 1, the classic architecture of a GSM type cellular network comprises a mobile switching centre (MSC) 106, a base station controller (BSC) 102 (and 103) and one or several base transceiver stations (BTS) 100 (and 101).
Each BTS provides the GSM radio coverage in one or several cells. For example, in relation to FIG. 1, the BTS 100 is controlled by the BSC 102 and covers the geographical cell C1, in which is located at least one user with a radio-communication mobile station (MS) T1.
More precisely, the MSC controls the configuration of the calls for each incoming or outgoing call, and acts as an interface with the other telecommunication networks. Each communication made with the MSC 106, which controls several BSC (102, 103) via TRAU code conversion and adaptor equipment (104, 105).
The BSC allocates the radio channels required for each call. It handles the intercellular transfers between two BTS′. A single BSC supports several BTS′ which cover a wide geographical zone.
Finally, the role of a BTS is to support the GSM radio transmission with the users of mobile stations. The BTS′ are located close to pylons which carry antennae, and are spread out in the geographical zone covered by the cellular network.
The GSM standard and its developments, as defined by the 3GPP group, use voice compression. This compression is carried out by TRAU (104, 105). According to the GSM standard, the TRAU (104, 105) may be implemented on the MSC site, on the BSC site or even on the BTS site. The financial considerations tend to have the TRAU (1204, 105) preferably implemented on the MSC site, in order to reduce the transmission costs.
Several types of codecs have been defined by the 3GPP group. The GSM FR (full rate) codec operates at a rate of 13 Kbit/s. The GSM HR (half rate) and EHR (enhanced full rate) respectively operate at 5.6 Kbit/s and 12.2 Kbit/s. After code conversion, speech at 64 Kbit/s compressed to 13/12.2 Kbit/s (respectively 5.6 Kbit/s) is transmitted to the BTS in a time slot of 16 Kbit/s (respectively 8 Kbit/s). According to the 3GPP TS 08.60 (respectively TS 08.61) specification, compressed speech is transmitted to the BTS every 20 ms according to the TRAU frame format.
These same principles apply to AMR (adaptative multi rate) FR and HR rates.
The TRAU frame transports, in addition to compressed speech information, “control bit” type information which allows the quality of the communications to be optimised between the code conversion entity and the encoding-decoding channel codec unit (CCU) of the BTS. These control bits especially allow the synchronisation of the information exchanged, to define the type of coding used (FR, EFR, HR or AMR) and also to indicate the discontinuity of the transmission due to the silences in the speech (DTX).
In order to introduce the implementation of a satellite connection in a cellular network, we will now describe briefly, in relation to FIG. 2, the interfaces used and their denomination between the main entities previously introduced.
The interface between a MSC (106) and a TRAU (104, 105) is called interface A.
The interface between a BSC 102 and the BTS 100 is called the Abis interface.
In the case where the TRAU 104 is implemented on the MSC site, the interface between the TRAU 104 and the BSC 102 is called Ater.
A satellite connection may be used in the transmission chain for each of these interfaces. The main problem of the insertion of a satellite connection onto one of these interfaces is therefore to determine how to transmit efficiently the information required whilst minimising the radio band required for the satellite transmission.
The Abis interface connects a BSC to a BTS and has one or several 2 Mbit/s connections (ITU G703/G704 standard). This is one of the interfaces that is classically implemented for satellite transmission.
This Abis interface transports traffic data, such as compressed speech and signal information.
On the Abis interface, there are two types of signal information which circulate:                signal messages exchanged with the BTS, transported in a specific signal channel, which allow the BTS equipment itself to be controlled as well as the mobile terminals (MS) that are in communication with it. The corresponding messages are specified by the GSM in the TS 08.58 specification;        in-band control information which is transmitted in the same flow as the traffic information. This information is transmitted in the TRAU frames. This information is composed of control bits, which are complementary to the data bits, and are explained in the TS 08.60/08.61 specifications.        
The first type of signal information; composed of protocol messages, are transported in dedicated time slots, with typically on the Abis interface a rate of 64 Kbit/s.
Each 2 Mbit/s connection of the Abis interface has 31 time slots (TS) that are allocated to signal channels or speech channels. Depending on the type of network and speech coding choices, a 2 Mbit/s connection on the Abis intervals may typically be used to support up to ten radio “transmission” access or transceiver (TRX) channels. Each TRX itself supports eight GSM full rate channels dedicated to speech or sixteen GSM half rate channels. The corresponding reservation for the speech channels on the Abis interface represents for each TRX an allocation of 2 TS at 64 Kbits/s (8*16 Kbits/s=16*8 Kbits/s=128 Kbits/s).
According to the size of the GSM network, the BTS is equipped with a number N of TRXs, which leads to a proportional occupation of the number of TS′ on the Abis interface.
2. Satellite Applications
In relation to FIGS. 2A and 2B, a classic GSM network implementing a satellite radio type connection will be described.
The GSM network thus usually comprises for each of the cells a MSC 106, a TRAU 104 (both in the NWK network), a BSC 102 and a base station BTS 100, which provides communication for the users with a mobile terminal T1 that are located in the coverage zone of the BRS 100.
Still in relation to FIGS. 2A and 2B, the set-up comprising the TRAU code converters (104, 105) and the MSC network communication centre(s) (106) in the NWK network form the core of the cellular network and is called the central connection network.
Furthermore, a radio connection LR is implemented on the Abis interface, between the BSC 102 and the BTS 100. This radio connection LR is for example provided by a satellite radio system containing emitter-receiver antennae on each side of the Abis interface, and a satellite 11.
It may be noted that in fact it is possible to insert a satellite radio connection on each of the interfaces implemented in the GSM system: A, Abis, Ater. However inserting such a satellite connection on the Abis interface, which is to say between the BSC and the BTS, is preferred to extend the GSM service to distant geographical locations with low user densities with minimal infrastructure costs.
In order to avoid any confusion, it is important to note that in such an implementation, two types of radio systems are used, but they do not have the same role:                the GSM network itself uses first radio connection means to communicate, and especially to make the transmission between the BTS and the mobile terminal users.        the satellite system consists of a second radio transmission connection. Usually, a device called a hub allocates the radio resources required for the transmission of the data by satellite between the BSC and the BTS.        
In fact, when two users are communicating, the usual set-up in a GSM network demands that the speech flow passes via the BSC, as well as via the TRAU and the MSC. This thus requires the allocation of resources on two channels with satellite connection: the uplink and the downlink. This remains especially true regardless of the position of the users (callers and receivers) without the possible proximity of the users being taken into account.
3. Known Loop Architecture
In relation to FIG. 2B, according to known architecture, a station adaptor CST 100A and a gateway adaptor CSG 102A for allowing the communications to be looped in a local loop zone. Local looping may thus be obtained by locally looping the communications of two terminals that are controlled by a same BTS or as is shown in FIG. 2B, in a zone controlled by a single station adaptor 100A. Within this known architecture, it is therefore possible not to transmit all of the data to the MSC 106 (nor to the BSC 102 nor to the TRAU 104, 105). Consequently the uplink connection network (intermediate network) that uses the LR radio connection, which allows bandwidth to be economised and the transit time for the data which composes the transmission to be reduced.
4. Disadvantages of the Prior Art
At present, the implementation of a radio connection, especially by satellite, between the BTS base stations and their corresponding BSC of a cellular network leads to, during a communication between two users each served by a different BTS connected by satellite, the allocation of two radio channels: a first channel for the receiver, and the second for the caller.
Indeed, the usual application requires that the speech “rises” up to the MSC of the GSM network. The speech flow then passes twice by the satellite (or by the backhaul network). A communication from a first cell controlled by a first BTS to a second cell controlled by a second BTS thus suffers needlessly from the addition of twice the satellite transfer time, which is around 250 milliseconds and the transmission of all of the data to their respective BSCs. The existence of this double satellite link and this data transmission to the BSCs thus adds not just a considerable transmission time which affects the quality of the communication perceived by the users, but is also very costly.
Consequently, at present there are no means which allow the nature of the proximity of a call from a first cell controlled by a first BTS to a second cell controlled by a second BTS to be specified. The current techniques thus do not manage such a configuration in an optimised manner.
In other terms, still in relation to FIG. 2A, a terminal T1, located in a zone covered by the BTS 100 that wishes to open a communication with a terminal T2, located in a zone covered by a BTS 101 uses the services of the satellite 11 twice via the LR radio connection. The data is transmitted by the BTS 100 to the BSC 102 and to the TRAU 104 that it belongs to. The TRAU 104 also communicates via a classic connection to a TRAU 105 and BSC 103 to which the BTS 101 belongs. Consequently, to establish the communication, the data transits to the BSCs and the TRAU, using the satellite radio link twice; once to go from the BTS 100 to the BSC 102/TRAU 104 and once to go from the TRAU 105/BSC 103 to the BTS 101.